The article suggests that he was "fired" in some way:
> Sources have said the move came amid growing
> tension between Sinofsky and other top executives.
> Sinofsky, though seen as highly talented, was
> viewed at the top levels as not the kind of team
> player that the company was looking for...
> In a press release, Ballmer praised Steven’s work,
> but also talked about a need for “more integrated
> and rapid development cycles for our offerings.”
Further, I think your parents' question still fits with a little bit of wrangling even if he wasn't fired. It would be interesting to know what it feels like to reach a very high rank at a company and then to step down.